The present invention relates to a process for forming a polymeric coating on the outside surface of a length of tubing and more particularly to tubing used as a packaging sheath for foodstuff such as cellulose hydrate tubing.
A process of the type concerned here is disclosed in German Auslegeschrift No. 2,124,685. However, in this known process, the outside surface of a length of plastic tubing is coated with an aqueous polymeric dispersion and because the tubing does not readily absorb water, the problem of marked water absorption and swelling, and of shrinkage on drying, such as arises with cellulose hydrate tubing, does not apply. In this prior publication, the plastic tubing is passed over rolls which approximately even out the thickness of the coating but cannot ensure precise metering. Further, the process disclosed in German Auslegeschrift No. 2,124,685 requires that the tubing be heated and cooled in the inflated state, which is relatively expensive.
German Pat. No. 1,142,297 discloses a process for coating the outside of cellulose hydrate tubing, wherein the outside of the tubing is exposed to an aqueous polymeric dispersion by passing the tubing through the dispersion. The tubing, coated with the aqueous dispersion, is then inflated with support air to the desired diameter, and is dried in this state. In the known processes there is the possibility of flaws forming in the coating due to the non-uniform run-off of the liquid coating layer resulting from the inflation of the tubing. The formation of coating layers having a uniform thickness over the entire outer circumferential surface of the tubing is frequently impossible, or only possible with great difficulty, when inflation of the tubing is required because virtually unavoidable pressure differences in the tubing cavity lead to non-uniform run-off of the relatively low-viscosity polymeric dispersion on the outside of the tubing, and hence, results in the formation of non-uniform coating. Moreover, the inflation of the tubing is necessary in the known processes to counteract the decrease in width of the tubing as a result of the tubing swelling due to the absorption of dispersing medium from the aqueous dispersion.
A further disadvantage of the known processes for externally coating cellulose hydrate tubing with an aqueous polymer dispersion of a customary concentration results from the use of aqueous coating dispersions having a relatively high water content because the cellulose hydrate tubing becomes heavily moistened with water, and tends to undergo an undesired change in dimension due to shrinkage, especially upon drying. Further, because the known processes employ an aqueous polymeric dispersion with a relatively low content of dispersed polymer, the amount of water which has to be expelled by the action of heat in order to form a solid film coating on the outside of the tubing is relatively large and requires a substantial expenditure of energy.
Furthermore, since the known processes require inflation of the tubing in order to counteract shrinkage, especially during the drying step, virtually unavoidable fluctuation of internal pressure in the tubing leads to the formation of uneven film coatings, resulting from the free run-off of the coating layer applied to the outside of the tubing.